1. Field of the Invention
The present invention relates to an image display medium repeatedly rewritable thereon, a particle for a display device using therein and an image forming apparatus.
2. Description of the Related Art
Traditionally, there have been proposed various display techniques as a image display medium repeatedly rewritable thereon, such as a Twisting Ball Display (bichromal particle rotational display) medium, an electrophresis medium, a magnetophoresis medium, a thermal rewritable medium, a liquid crystal medium having a memorizing property and the like. There has been a problem that the display techniques described above cannot obtain a white display like paper on a display screen and have a low contrast in a display thereon, though with excellency in image memorizing property.
As a display technique, using a toner, to solve a problem as described above, a technique has been proposed in which a conductive colored toner and white particles are sealed between electrode substrates facing each other, a charge is injected into the conductive colored toner through a charge transporting layer provided on the inner surface of an electrode on a non-display substrate, and the conductive colored toner injected with a charge is moved to the display substrate facing the non-display substrate by an electric field between the electrode substrates and attached to the inner side of the display substrate to thereby present an image display with the help of a contrast between the conductive colored toner and the white particles (see “Japan Hardcopy '99, collection of papers, pp. 249–252”). In this technique, an image display medium is all constituted of solid constituents and excellent in that displays in white and black, in principle, can be perfectly switched therebetween. In the above described technique, however, there exist a conductive colored toner not in contact with the charge transporting layer provided on the inner side surface of an electrode on the non-display substrate and/or a conductive colored toner isolated from the other conductive colored toner, and because these toners are not injected with charge, are not moved by the electric field and randomly distributed on the display substrate. Therefore, there arises a problem that a density contrast is lowered.
An image display medium has been proposed for the purpose to solve such a problem, which includes a pair of substrates and particle groups of plural types of particles different in color and electrical charge characteristic from each other sealed between the substrates so as to be freely movable between the substrates by an applied electric field (see “Japanese Patent Application Laid-Open (JP-A) No. 2001-312225”). With this image display medium, a high whiteness and a high density contrast can be attained.
Though image display medium is excellent in a white density and a black density in an initial stage, after repetition of rewriting, reduction occurs in image density, density contrast and uniformity of an image, thereby causing unevenness in image.